ABSTRACT

The first fibre optic system for permanent monitoring of downhole pressure and temperature has been successfully installed In an onshore gas well for Nederlandse Aardolie Maatschapplj B V (NAM) In the Sleen field In the Netherlands The system is design for 1000 bar pressure and 200 °C temperature, and has been fully qualified A complete system has been developed comprising passive gauge, downhole cable, penetrator unit for the wellhead, wellhead and pod connectors (for subsea applications), optoelectronics for sensor operation, and conversion and logging unit.

INTRODUCTION

Early and reliable data of downhole pressure and temperature is required in order to enable cost effective production, initially to refine and tune the reservoir models and subsequently to optimise production and decide on possible maintenance work Tools available to acquire downhole pressure and temperature information can be divided In two main technology groups, e wireline techniques and permanent downhole installations, where each of the two approaches has its specific characteristics (1)

- Wireline techniques

  • Temporary installation, i e limited reliability requirement

  • Special equipment and special service personal required for each survey

  • Increased activity at the well imposes higher possibility of well damage during the survey

  • High cost per survey

- Permanent downhole installation

  • Long mission tune, e high reliability requirement

  • Information available on request

  • No involvement of specla1 personnel at the well site, i e. increase safety aspect

  • No intervention cost for surveys

Comparison of the listed characteristics shows the advantages of the permanently installed systems In order to make this option applicable, the high reliability required for permanent installations should be realised In a practical system

FIELD EXPERIENCE WITH ELECTRICAL DHM SYSTEMS

Since 1987 specially developed electrical downhole monitoring (DHM) systems became available and permanent installations were made In the North Sea Typically these systems include downhole installed sensors and electronics (200 to 300 components), electric cabling, splices, wellhead penetration and wellhead feed through, and remote electronics for powering and reading the downhole gauge system

Data has been accumulated for subsea installations In the North Sea, being realised between 1987 and 1993, Fig I. Some installations were made In reservoirs with temperatures up to 120°C, but most were at temperatures around 100°C Results indicate that 20% to 25% of the systems failed directly after the installation, whereas more than 50% failed within two years, this failure probability is high in relation with the current practices on planned intervention work, typically once every 5 years

From those installations were failure modes could be determined, it appeared that roughly 1/3 was related to the downhole sensors and electronics, 1 / 3 was related to the cable and 1 / 3 to connections

For future DHM applications two additional aspects will affect the reliability requirement for permanently installed systems and reduce the applicability of electrical DHM systems, i e

  • Extension of the period between interventions to up to 10 years, enlarging the discrepancy between reliability target and the current reliability performance of electrical systems

  • New applications In deeper reservoirs will have higher pressure and higher temperature characteristics

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